The measurement of dissolved oxygen or oxygen-partial pressure or tension in fluids including gases and liquids is typically done with nonportable equipment utilizing electrodes for the oxygen measurement. This equipment is fairly expensive, and the procedures for its use can be cumbersome depending on the type of fluid to be measured. For instance, when the fluid is blood that is to be tested for blood gases, the blood sample is drawn in a syringe, immersed in ice and transported quickly to the lab where the equipment is usually located for a measurement of the gases including oxygen.
In these gas measuring machines the electrodes that are used to assist in sensing the oxygen normally consist of some configuration of the Clark cell or Clark electrode. The Clark cell normally consists of two electrodes, an anode and a cathode, both of which typically are formed by embedding a small wire, typically made of silver, gold or platinum into an insulating housing of glass or plastic to be used as the cathode and to surround the cathode with a ring or another wire typically made of silver to be used as the anode or reference electrode. The cathode and anode are in ionic contact typically by way of an ionic conducting electrolyte solution. The electrolyte is typically sealed into a chamber that holds the platinum wire and the reference electrode by means of a hydrophobic, gas permeable membrane that covers the whole assembly. This membrane serves two functions. It allows diffusion of the oxygen to the cathode surface. Also it provides a sensor with selectivity in restricting passage of materials which could possibly contaminate the cathode surface and interfere with the oxygen reduction reaction. The Clark electrode integrates the cathode and the reference electrode into a single unit. A voltage of about 0.5 to 0.8 volts is applied between the platinum wire and a reference electrode, which is also located in the electrolyte. With the platinum wire having a negative voltage with respect to the reference electrode, a reduction of the oxygen takes place at the platinum cathode. As a result, an oxidation-reduction current that is proportional to the partial pressure of the diffused oxygen is measurable. This current in a simple form is approximately I=nFAf, where n is equal to the number of electrons participating in the reduction, F is the Farady constant, A is the electrode surface area, and f is the oxygen flux to the electrode surface. These Clark electrodes assist in measuring gases like oxygen in fluids by their connection to the nonportable machines for the necessary electronics to obtain a read out of the value measured. They are placed in contact with the fluid to be measured, for instance blood, by placement in a cuvette of blood for in vitro measurement of the oxygen tension in the blood, or placement at the tip of a catheter for insertion into various parts of the body for in vivo measurements of oxygen tension in the blood.
Efforts have been made recently to provide more portable devices that shorten or overcome transporting the sample to the measuring machine at a fixed location. For example, portable sensing units which can be coupled to a digital readout device would be useful at the patient's bedside in a manner similar to a way that temperatures are measured at the patient bedside.
U.S. Pat. Nos. 3,000,805 and 3,497,442 show two such devices. The former has electrodes located on a syringe plunger and the latter has electrodes placed on the syringe well to conduct the measurements. The electrodes of these sensors may be particularly sensitive to small sample volumes since they consume oxygen in their operation. In the U.S. patent application Ser. No. 07/343,234, Applicants assignee describes and claims a portable blood gas sensor which includes an oxygen sensor fabricated using a conventional silk screening process where the electrodes are screened on to a ceramic substrate. The silk screening process, while effective in preparing electrical conductors, is limited in its ability to produce reproducible electrode surface area from batch to batch. The result is the production of sensors that give outputs that varies over a wide range even when the oxygen tension is a constant. Also, the large cathodes from the silk screening process consume amounts of oxygen in operation, and this degree of consumption makes stable measurements of small quantities of oxygen in the sample more difficult.